X-ray diagnostic apparatuses detect X-rays transmitted through a subject by using an X-ray detector to acquire X-ray images. Although dominant X-ray detectors are planar detectors that use the thin film transistor (TFT) technology of liquid crystal displays, there are also X-ray detectors that use couple charged device (CCD) image sensors, complementary metal oxide semiconductor (CMOS) image sensors, or the like. Here, in X-ray images acquired by using an X-ray detector of the CCD or CMOS, point-like noise (white noise) with a large pixel value sometimes occur in an unexpected manner on a pixel to pixel basis. Specifically, if a CCD or CMOS is used for an X-ray detector, an atom transitions in the CCD or CMOS due to incident X-ray, and the energy emitted from the transitioned atom is detected, which results in noise.
Furthermore, X-ray diagnostic apparatuses perform image processing on acquired X-ray images to reduce noise. For example, the X-ray diagnostic apparatus reduces noise due to a filtering process, during which the degree of similarity between pixels included in an X-ray image is calculated and weighted averaging is conducted on pixels by using the calculated degree of similarity. Such a filtering process includes, for example, a noise reduction process using a filter (hereafter, referred to as a coherent filter) disclosed in Japanese Patent No. 4170767, a non-local means (NLM) filter, or the like. The noise reduction process using the above filters makes it possible to improve the image quality of X-ray images by decreasing noise while the occurrence of blurring is reduced.
However, if an X-ray image includes white noise, the white noise sometimes remains even after a filtering process. Specifically, the calculated degree of similarity between the pixel, which is white noise, and a different pixel is low, and a weight used for weighted averaging is small; as a result, even if weighted averaging is conducted, the pixel value is not so changed, and white noise still remains. Furthermore, in X-ray images after a noise reduction process, noise other than white noise is reduced and, as a result, white noise becomes more noticeable in some cases.